1. Field of the Invention
The present invention relates to detection of a concentration of hydrogen peroxide vapor in a treatment vessel, and more particularly to a method and apparatus for detection of the concentration of hydrogen peroxide vapor by means of a semiconductor gas sensor, in a system for such treatments as sterilization and disinfection, by bringing an object for treatment into contact with hydrogen peroxide vapor in a treatment vessel, with at least the pressure kept at a constant level.
2. Description of the Prior Art
Since hydrogen peroxide vapor decomposes itself into harmless oxygen and water when coming into contact with a solid and generates nascent oxygen with sterilization and other effects, it can be used as a sterilizing agent and disinfectant and offers a wide range of application possibilities. It is currently used in sterilizing and disinfection treatments of such objects as pharmaceutical basic materials, pharmaceutical end products, and food packaging, in the pharmaceutical, medical supply and equipment manufacturing, and food industries. In such treatments, hydrogen peroxide vapor in a specific concentration is supplied under a certain pressure, generally under atmospheric pressure, into a treatment vessel storing therein objects to be treated, for example, packaging materials placed therein, to sterilize and disinfect those objects by bring them into contact with hydrogen peroxide vapor.
The problem is, however, that the concentration of hydrogen peroxide vapor drops with time in the treatment vessel because it is decomposed and consumed when the vapor is in contact with the object to be treated and the inner wall of the vessel itself. Sterilization and disinfection with hydrogen peroxide utilizes nascent oxygen which is generated from a decomposition of hydrogen peroxide. Sterilization and disinfection thus becomes ineffective when the concentration of hydrogen peroxide vapor falls below a certain level. Yet, it would be quite wasteful to supply too much hydrogen peroxide vapor into the vessel and keep the concentration higher than necessary for sterilization and disinfection. In addition, disposal of excess hydrogen peroxide vapor would add a cost burden, because it would be necessary to enlarge facilities for treating the excess vapor before releasing it into the atmosphere.
To perform sterilization and disinfection efficiently, the concentration of hydrogen peroxide vapor in a treatment vessel has to be controlled and maintained in a proper range. If such concentration control is to be exercised properly, it is essential to detect precisely and in real time the concentration of hydrogen peroxide in the vessel.
Among the known methods of directly detecting the concentration of hydrogen peroxide are the controlled potential electrolysis, test-paper photoelectric photometry, and detection tube methods. None of these allows real-time detection of the concentration of hydrogen peroxide vapor, however. Accordingly, none of these is suitable for detection of the concentration of hydrogen peroxide vapor in such treatments as sterilization and disinfection.
The controlled potential electrolysis method uses a concentration detector comprising a working electrode and a counter electrode arranged in a region for containing the electrolytic solution, isolated from the outside by a partition. As the hydrogen peroxide vapor penetrates the partition and diffuses into the region for containing the electrolytic solution, it is adsorbed on the working electrode comprising an electrochemical catalyst, and causes an oxidation or reduction reaction to generate electric current between the electrodes, which is measured, thereby detecting the concentration of hydrogen peroxide vapor. The problem with this method is that the hydrogen peroxide vapor, that is, the gas to be detected which has diffused into the electrolytic solution-containing region, will be adsorbed by and remain in the electrolytic solution after detection is over (i.e., after the sterilization operation is completed with the hydrogen peroxide vapor removed from within the treatment vessel) and the hydrogen peroxide by the working electrode will not be removed for a long time. That is especially the case where the concentration of hydrogen peroxide vapor applied is high enough for effective sterilization and disinfection, not lower than 500 ppm, for example. The use of the vapor in very low concentrations, 10 ppm, for example, does not present this problem. With the working electrode in such condition, the concentration detector is very low in sensitivity to the change in the concentration of hydrogen peroxide and cannot determine the concentration of hydrogen peroxide in real time and accurately.
The test-paper photoelectric photometry method is carried out by a concentration detector with test paper incorporated therein, the test paper which is given a special treatment with a chemical so as to color upon contact with the hydrogen peroxide vapor. By measuring the intensity of the coloring of the test-paper, the concentration of hydrogen peroxide vapor is detected. The test paper contains, in addition to the coloring chemical, a certain amount of water to facilitate the coloring. The trouble is that when it comes into contact with the moisture contained in the test paper, the hydrogen peroxide vapor is dissolved in the water, resulting in a changed concentration of hydrogen peroxide vapor around the concentration detector, which makes accurate detection virtually impossible. Meanwhile, the object to be treated is often put into the treatment vessel and dried, because if moisture is on the object to be treated in sterilization and disinfection, the hydrogen peroxide vapor is dissolved, thereby reducing the effectiveness of hydrogen peroxide vapor sterilization of the object to be treated. But if the object is dried in the vessel, the test paper in the concentration detector mounted therein will be dried as well and the moisture contained in the test paper evaporates out of the test paper. Thus, accurate detection of the hydrogen peroxide vapor is virtually impossible with such test paper. Furthermore, there is a concern that the chemical contained in the test paper can stain the object to be treated when it evaporates. As pointed out, the test paper photoelectric photometry method is not suited and can not be adopted as method for detection of the concentration of hydrogen peroxide vapor in sterilizations and disinfections that have to be performed in a low-humidity atmosphere.
The detection tube method utilizes a concentration detector with a glass tube filled with a detector agent which undergoes a chemical reaction and changes in color on contact with hydrogen peroxide vapor in the tube. The idea is that as the hydrogen peroxide vapor is led into the glass tube through the mouth of the tube, the detector agent changes in color. The length of the color change is measured by a scale on the glass tube to determine the concentration of hydrogen peroxide vapor. But this method does not permit continuous detection of concentration and can not determine the concentration of hydrogen peroxide vapor real-time.
Having determined that the semiconductor sensor widely used for the detection of the concentration of H.sub.2, CO, alcohol, and other chemicals could be applied to detection of hydrogen peroxide vapor, the present inventors have developed a method for detection of hydrogen peroxide (this shall be called prior art in the present specification) comprising providing a semiconductor gas sensor inside a treatment vessel with a concentration indicator outside, so that the output of the semiconductor gas sensor was converted into the concentration of hydrogen peroxide vapor and the measurement was indicated on the concentration indicator.
The aforesaid semiconductor gas sensor generally comprises a sensor element made of sintered metal oxide, electrodes embedded therein, and a means for heating the sensor element (indirect or direct heating type). The principle of this prior art method is this: when the gas constituents are adsorbed on the surface of such oxide particles as n-type semiconductor oxide and p-type semiconductor oxide, the free electrons around the surface move to change the electro-conductivity. This change in electro-conductivity is detected. If the hydrogen peroxide is chemically adsorbed by the oxide semiconductor element in the treatment vessel, the free electrons move in the element, increasing the electro-conductivity of the element. This reaction takes place in a very short time. That is, the sensor output quickly reflects the change in concentration.
Meanwhile, the concentration indicator, to which the output signals are constantly input from the semiconductor gas sensor, indicates the hydrogen peroxide vapor concentration, a converted value of the output of the semiconductor gas sensor. The conversion rate is set beforehand on the basis of data from an experiment representing the relation between the sensor output and the concentration of the hydrogen peroxide vapor. The experiment is conducted in accordance with the following procedure: the aqueous solution of hydrogen peroxide having the same properties as the one to be used in sterilization and disinfection is injected by micro syringe into a closed experiment vessel (heat-resistant vessel) filled with a clean atmosphere and is completely evaporated by heating instantly by a heater or the like. With the temperature and humidity inside the experiment vessel maintained at constant level, measurements are taken of the output of the semiconductor gas sensor mounted in the experiment vessel. That is, the decrease in resistance is converted into the increase in voltage as in an electric circuit. The quantity of aqueous hydrogen peroxide injected is varied, and the outputs of the sensor are measured at different levels of the amounts as injected, and thus the relation between the output of the sensor and the concentration of hydrogen peroxide is obtained. The concentration of hydrogen peroxide in the experiment vessel can be calculated from the volume of the experiment vessel and the concentration and quantity of the aqueous solution of hydrogen peroxide injected. The correlation thus obtained is always constant, and from this correlation can be obtained a constant conversion rate.
By this prior art, as described, the output of the semiconductor gas sensor can be converted at a constant conversion rate and shown on a concentration indicator. Thus, the concentration of hydrogen peroxide vapor in the treatment vessel can be known on a real-time basis. That is to say, the concentration of hydrogen peroxide vapor can be well controlled, and sterilization and disinfection operations can be carried out efficiently.
It was found, however, that the concentration determined by this prior art method could differ from the actual level of the hydrogen peroxide vapor in the treatment vessel and that no accurate measurement could be made of the concentration of the hydrogen peroxide vapor by the semiconductor gas sensor.
In an effort to solve this problem, the inventors conducted various experiments and researches. It was discovered that even if the concentration of hydrogen peroxide vapor was kept at a constant level in the treatment vessel, the output of the sensor changes with changes in the temperature or humidity in the treatment vessel. The conversion rate for turning the output of the sensor into the hydrogen peroxide concentration was one based on the data worked out with the temperature and humidity maintained at a constant level in the experiment vessel as described. It was found that, in the actual sterilization and disinfection treatments in which temperatures and humidities are varied, the value obtained by converting the sensor output at the aforesaid conversion rate could be different from the actual hydrogen peroxide vapor concentration. The prior art method could not control the concentration correctly in such treatments as sterilization and disinfection, which in practice are performed under varying temperature or humidity conditions.